An optical imaging method is known as a method for noninvasively visualizing the information inside a living body. According to the optical imaging method, signals, such as acoustic waves and fluorescence, emitted from a substance (light-absorbing body) which absorbed light in an object to be measured when the object to be measured is irradiated with light are measured, and then imaged. By giving a pigment absorbing light in a near-infrared wavelength region, such as Indocyanine green (hereinafter sometimes abbreviated as ICG), into a living body, fluorescence or acoustic waves emitted from the pigment in the living body can also be measured. Therefore, pigments, such as ICG, can be used as a contrast agent for optical imaging.
Herein, in order to use a pigment as a contrast agent for tumors, it is suitable to bond a polymer to the pigment. Since the pigment generally has a low molecular weight, the pigment is promptly discharged from blood before accumulated in a tumor. However, by bonding a polymer, the pigment is hard to be discharged. Therefore, the pigment is accumulated in a tumor while circulating in blood, and, consequently, the accumulation amount in the tumor can be increased.
On the other hand, when the signal intensity from a portion other than a tumor site is lower than the signal intensity generated from the tumor site, a tumor portion can be visualized with high contrast. Therefore, it is desirable that the pigment does not remain for a long time in tissues other than the tumor portion, particularly blood vessels spreading in the entire living body. More specifically, it is desirable that a ratio (Tumor/Blood ratio, which is hereinafter sometimes abbreviated as T/B) of the accumulation amount in the tumor to the retention amount in blood of the contrast agent is high.
PCT Japanese Translation Patent Publication No. 2012-520856 (hereinafter referred to as PTL 1) describes a compound obtained by causing monoamine-PEG20k to react with Cy5 (Registered Trademark)-Bis NHS ester as an optical imaging agent. In the specification, k represents 1000 and, for example, 20 k is 20000. PEG20k herein means that the average molecular weight of PEG is 20000. In the specification, PEG is the abbreviation for polyethylene glycol. More specifically, a compound is disclosed in which the accumulation properties in tumor is increased by bonding two high molecular weight polymers (PEG) of 20 k to a pigment (Cy5 (Registered Trademark)).